three-mile belt

from The Collaborative International Dictionary of English v.0.48

Three-mile \Three"-mile`\, a.
   Of or pertaining to three miles; as,

   {the three-mile limit}, or the limit of the marine belt (the

   {three-mile belt} or {three-mile zone}) of three miles
      included in territorial waters (which see) of a state.
      [Webster 1913 Suppl.]
      [1913 Webster]